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Summary: Methods

# Future

`public interface Future`
``

java.util.concurrent.Future<V>

Known indirect subclasses CompletableFuture<T>, CountedCompleter<T>, ForkJoinTask<V>, FutureTask<V>, RecursiveAction, RecursiveTask<V>, RunnableFuture<V>, RunnableScheduledFuture<V>, ScheduledFuture<V>
CompletableFuture<T> | A `Future` that may be explicitly completed (setting its value and status), and may be used as a `CompletionStage`, supporting dependent functions and actions that trigger upon its completion.
CountedCompleter<T> | A `ForkJoinTask` with a completion action performed when triggered and there are no remaining pending actions.
ForkJoinTask<V> | Abstract base class for tasks that run within a `ForkJoinPool`.
FutureTask<V> | A cancellable asynchronous computation.
RecursiveAction | A recursive resultless `ForkJoinTask`.
RecursiveTask<V> | A recursive result-bearing `ForkJoinTask`.
RunnableFuture<V> | A `Future` that is `Runnable`.
RunnableScheduledFuture<V> | A `ScheduledFuture` that is `Runnable`.
ScheduledFuture<V> | A delayed result-bearing action that can be cancelled.

A `Future` represents the result of an asynchronous computation. Methods are provided to check if the computation is complete, to wait for its completion, and to retrieve the result of the computation. The result can only be retrieved using method `get` when the computation has completed, blocking if necessary until it is ready. Cancellation is performed by the `cancel` method. Additional methods are provided to determine if the task completed normally or was cancelled. Once a computation has completed, the computation cannot be cancelled. If you would like to use a `Future` for the sake of cancellability but not provide a usable result, you can declare types of the form `Future<?>` and return `null` as a result of the underlying task.

Sample Usage (Note that the following classes are all made-up.)

     interface ArchiveSearcher { String search(String target); }
     class App {
       ExecutorService executor = ...
       ArchiveSearcher searcher = ...
       void showSearch(final String target)
           throws InterruptedException {
         Future<String> future
           = executor.submit(new Callable<String>() {
             public String call() {
                 return searcher.search(target);
             }});
         displayOtherThings(); // do other things while searching
         try {
           displayText(future.get()); // use future
         } catch (ExecutionException ex) { cleanup(); return; }
       }
     }

The `FutureTask` class is an implementation of `Future` that implements `Runnable`, and so may be executed by an `Executor`. For example, the above construction with `submit` could be replaced by:

     FutureTask<String> future =
       new FutureTask<>(new Callable<String>() {
         public String call() {
           return searcher.search(target);
       }});
     executor.execute(future);

Memory consistency effects: Actions taken by the asynchronous computation happen-before actions following the corresponding `Future.get()` in another thread.

See also:

- `FutureTask`
- `Executor`

## Summary

### Public methods
`abstract boolean` | `cancel(boolean mayInterruptIfRunning)` Attempts to cancel execution of this task.
`abstract V` | `get(long timeout, TimeUnit unit)` Waits if necessary for at most the given time for the computation to complete, and then retrieves its result, if available.
`abstract V` | `get()` Waits if necessary for the computation to complete, and then retrieves its result.
`abstract boolean` | `isCancelled()` Returns `true` if this task was cancelled before it completed normally.
`abstract boolean` | `isDone()` Returns `true` if this task completed.

## Public methods

### cancel

    public abstract boolean cancel (boolean mayInterruptIfRunning)

Attempts to cancel execution of this task. This attempt will fail if the task has already completed, has already been cancelled, or could not be cancelled for some other reason. If successful, and this task has not started when `cancel` is called, this task should never run. If the task has already started, then the `mayInterruptIfRunning` parameter determines whether the thread executing this task should be interrupted in an attempt to stop the task.

After this method returns, subsequent calls to `isDone()` will always return `true`. Subsequent calls to `isCancelled()` will always return `true` if this method returned `true`.

Parameters
`mayInterruptIfRunning` | `boolean`: `true` if the thread executing this task should be interrupted; otherwise, in-progress tasks are allowed to complete

Returns
`boolean` | `false` if the task could not be cancelled, typically because it has already completed normally; `true` otherwise

### get

    public abstract V get (long timeout,
                    TimeUnit unit)

Waits if necessary for at most the given time for the computation to complete, and then retrieves its result, if available.

Parameters
`timeout` | `long`: the maximum time to wait
`unit` | `TimeUnit`: the time unit of the timeout argument

Returns
`V` | the computed result

Throws
`CancellationException` | if the computation was cancelled
`ExecutionException` | if the computation threw an exception
`InterruptedException` | if the current thread was interrupted while waiting
`TimeoutException` | if the wait timed out

### get

    public abstract V get ()

Waits if necessary for the computation to complete, and then retrieves its result.

Returns
`V` | the computed result

Throws
`CancellationException` | if the computation was cancelled
`ExecutionException` | if the computation threw an exception
`InterruptedException` | if the current thread was interrupted while waiting

### isCancelled

    public abstract boolean isCancelled ()

Returns `true` if this task was cancelled before it completed normally.

Returns
`boolean` | `true` if this task was cancelled before it completed

### isDone

    public abstract boolean isDone ()

Returns `true` if this task completed. Completion may be due to normal termination, an exception, or cancellation -- in all of these cases, this method will return `true`.

Returns
`boolean` | `true` if this task completed

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